U.S. patent application Ser. No. 610,947 filed May 16, 1984 entitled "Solid State Current Limiting Circuit Interrupter" in the name of E. K. Howell discloses the use of semiconductor elements in combination with circuit interrupting contacts to allow the contacts to separate without the occurrence of an arc between the contacts. In the Howell application, which is incorporated herein for purposes of reference, a transistor element is employed in combination with a voltage dependent resistor to transfer the current away from the separating contacts to the transistor and thence from the transistor to the voltage dependent resistor. Some means is required for switching the transistor between conductive and non-conductive states in order for the transistor to be conductive when the contacts are first opened and for the transistor to become non-conductive shortly after contact separation. The Howell application advantageously employs a saturable core current transformer for switching the power transistor on and off within predetermined time intervals. It has since been determined that the same function which the transistor performs can be accomplished by means of a resistor fabricated from a positive temperature coefficient material (PTC) having a relatively low resistance value at low temperatures and a substantially higher resistance at a predetermined higher temperature.
U.S. Pat. Nos. 4,329,726 and 4,413,301 to L. M. Middleman et al. disclose PTC materials operational in the range of 5 to 100 amperes which are employed in series with separable contacts in order to provide circuit protection by the increased series resistance within the circuit when the PTC material carries current higher than a predetermined value.
The use of a material having a negative temperature coefficient within circuit interrupting devices is disclosed within U.S. Pat. No. 4,019,097 entitled "Circuit Breaker With Solid State Passive Overcurrent Sensing Device." This patent teaches the use of a material such as vanadium dioxide or lanthanum cobalt oxide in series with a flux transfer trip mechanism. The thermal response properties of the aforementioned materials are used to sense the presence of an overcurrent condition and to allow the current through a trip mechanism to increase to an operational value. All the aforementioned patents are incorporated herein for purposes of reference. The materials described within the patents to Middleman et al. are incapable of carrying sufficient current to provide overcurrent protection in a circuit such as protected by a molded case circuit breaker.
The purpose of the instant invention is to provide a fault current interrupter employing positive temperature coefficient resistors within circuits capable of interrupting current within residential and industrial power buses without becoming damaged or destroyed in the process.